Has Anyone Ever Run TWO Identical Pairs of Speakers ?

mijostyn - You are absolutely correct in theory and the higher the frequency, the more it matters. My question is have you tried it? Sometimes the benefits outweigh the image issue. Just my 2 cents.

In the early 1970's, I ran stacked Advents (tweeters adjacent), powered by a Phase Linear 400. These speakers (and stacked was popular), were flat enough to not add too much of anything anywhere in their frequency range.   A decade later, I ran the Phase linear through stacked B&W DM 14's (Also tweeter adjacent) for many years (25?), including years that I also owned Acoustats or Maggies.  It works well with speakers that do not accentuate anything too  much, but it is a horrible thing to do with some speakers that do have noticeable peaks and valleys. 

you are talking about an old thing people used to do
 one example is Quad   many others also  Now we have   MTM  type speakers
  I would not go looking for any  unless you trip over a pair you like     

Fiesta, it is not theory. It is solid acoustic management. Yes, I have done it. I have two pair of Mirage speaker in my workshop. I got them in 1987 as stop gap speakers while I was waiting for my Apogee Divas to be made.  I tried stacking them one on top of the other with the top speakers upside down trying to get the tweeters as close together as possible. It went plenty loud. Low bass was missing and the imaging was vague. To get more midrange power a D'Appolito array is the best way to go if you have a strong enough tweeter. You could do a linear array of tweeters stacking small tweeters in a vertical line ala Pipe Dreams otherwise a single tweeter is always best which is why so many very expensive speakers do just that like Magico, Wilson and YG. This is my chief argument with Tekton speakers. Tweeter arrays like they use may get cheap tweeters to go loud but imaging goes right down the tubes. They might pay $2.50  for each one vs $300 to $400 for real tweeters. Real speaker designers are laughing their a---- off. You have already seen multiple Tekton buyers relating that they are very unhappy with the purchase. 

I'm certainly not saying multiple tweeters are the way to go, in fact I like both vertical and horizontal driver alignment best. Just said sometimes the benefit outweighs the imaging issue, like volume when I was still in college.